Electrostatographic charging apparatus

ABSTRACT

A corona generating apparatus is provided comprising a back-up plate and screen wires, at least one corona discharge wire positioned therebetween, adapted to be mounted in closely spaced relation with an electrostatographic plate to uniformly apply an electrostatic charge onto said plate, a corona-generating potential source connected to said corona discharge wire adapted to generate a corona discharge from said corona wire, and a second potential source adapted to be connected to said screen wires and impress a time or position varying voltage upon said screen wires permitting high initial charging current to flow thereby rapidly charging said plate while limiting the ultimate charge applied thereto.

United States Patent Schneider et al.

[ Aug. 29, 1972 [54] ELECTROSTATOGRAPHIC CHARGING APPARATUS [73]Assignee: Xerox Corporation, Rochester, NY.

[22] Filed: Oct. 26, 1970 [2l] Appl. No.: 84,005

[56] g References Cited UNITED STATES PATENTS 9/1970 Culhane ..250/49.5ZC 2/1970 Cunningham ..250/49.5ZC

Primary ExaminerArchie R. Borchelt Assistant Examiner-C. E. ChurchAttorneyJames J. Ralabate, William Kaufman and Barry Kramer ABSTRACT Acorona generating apparatus is provided comprising a back-up plate andscreen wires, at least one corona discharge wire positionedtherebetween, adapted to be mounted in closely spaced relation with anelectrostatographic plate to uniformly apply an electrostatic chargeonto said plate, a corona-generating potential source connected to saidcorona discharge wire adapted to generate a corona discharge from saidcorona wire, and a second potential source adapted to be connected tosaid screen wires and impress a time or position varying voltage uponsaid screen wires permitting high initial charging current to flowthereby rapidly charging said plate while limiting the ultimate chargeapplied thereto.

9 Claims, 2 Drawing Figures PATENTEflnuczs m2 SHEET 1 [IF 2 Illa.

John M35255??? PATENTEDmszs I972 3. 688, 1 07 sum 2 or 2 wwsELECTROSTATOGRAPHIC CHARGING APPARATUS This invention relates toelectrostatography. More particularly this invention relates to improvedelectrical circuitry for controlling a corona generating device forrapidly and uniformally applying electrostatic charge onto anelectrostatographic plate.

The basic electrostatographic process is disclosed in the Carlson U.S.Pat. No. 2,297,691. In this process an electrostatographic platecomprising a photoconductive insulating material on a conductive backingis given a uniform electric charge over its surface and is then exposedto the subject matter to be reproduced usually by conventionalprojection techniques. This exposure discharges the plate areas inaccordance with the radiation intensity which reaches them and therebycreates an electrostatic latent image on or in the plate coating whichmay then be developed with an electroscopic material whichelectrostatically clings to the plate in a visual pattern correspondingto the electrostatic image. Thereafter the developed image is usuallytransferred to a support material to which it may be fixed by anysuitable means.

The charging of the electrostatographic plate in preparation for theexposure step can be accomplished by means of a corona generating devicewhereby electrostatic charge is applied to the electrostatographic plateto raise it to a potential of approximately 500 to 600 volts. One formof corona generating device for this purpose is disclosed in the WalkupU.S. Pat. No. 2,777,957, wherein a plurality'of parallel wires areconnected in series to a high voltage source and are supported in aconductive shield that is arranged in closely spaced relation to thesurface to be charged. When the wires are energized, corona is generatedalong the surface of the wires and ions are caused to be deposited onthe adjacent photoconductive surface. Suitable means are usuallyprovided to effect relative movement of the surface to be charged andthe corona generating device. A biased wire shield placed between thecorona wires and the electrostatographic plate permits energizing thecorona wires to a potential well above the corona .threshold potentialthereof without causing damage to the electrostatographic plate becausethe excess of corona current over that required for proper charging ofthe plate is drained off by the biased shield. This type of coronagenerating device is referred to in the art as a scorotron.

As is well known, the corona threshold potential and the corona currentfrom an energized wire are functions of the wire diameter, i.e., thecorona threshold increases and the corona current for any givenpotential decreases as the wire diameter is increased. Variations in thepotential applied to corona wires of a given diameter will causerelatively large changes in corona current with corresponding variationsin the charging rate. In addition, the corona threshold potential andcorona current are also affected directly by deposits of dust that mayaccumulate on the wire and by variations of movement and ionizedconditions of the air sheath surrounding the wire. Thus, when operatingat the corona threshold, minute differences in wire diameter, slightaccumulations of dust on the wire and variations in air current or inair pressure drastically affect the corona generating potential of thewire causing nonuniform electrostatic charge to be deposited on theelectrostatographic plate.

It has heretofore been established that consistent high qualityreproductions can best be obtained when uniform potential is applied tothe electrostatographic plate in preparation of the plate for theexposure step. If the electrostatographic plate is not charged to asufficient potential, the electrostatic latent image obtained uponexposure will be relatively weak and the resulting deposition of anelectroscopic material thereon will be correspondingly small. If,however, the electrostatographic plate is overcharged, the converse willoccur and if overcharged sufficiently, the photoconductive layer of theelectrostatographic plate can be permanently damaged.

Since the contrast value, comparable to the contrast values obtainedfrom silver halide papers, of the electrostatic latent image may berelated directly to the potential charge on the electrostatographicplate before exposure, it is apparent that if the plate is not uniformlycharged over its entire area, the contrast value of the electrostaticlatent image obtained upon exposure will vary in different areas on theplate, and a mottled effect will be visible on the image when developed.

An improved scorotron device whereby a uniform electrostatic charge canbe deposited on the electrostatographic plate is disclosed in theCodichini U.S. Pat. No. 3,062,956. The scorotron device describedtherein consists of a backup plate, corona generating wires called thecoronode, and screen wires. The coronode fires, by corona discharge,charging the photoconductive surface of the electrostatographic plate.The potential applied to the plate surface is varied by changing thescreen potential. To ensure a constant charging current during operationat any given contrast setting, the charging circuit contains a currentstabilizer and a regulated direct current power supply. The currentstabilizer is a device for controlling charging current by automaticallyadjusting the screen potential when a current change is sensed.

In operation, any change in the charging current from the coronodes tothe electrostatographic plate produces a change in the applied voltageto the grid of a control tube, for example, a high gain pentode, theoutput of said control tube being applied to the screen wires of thescorotron device. In this manner, any change in the charging currentfrom the coronodes to the electrostatographic surface results in achange in the control tube resistance which, in turn, produces a changein the screen potential. With this circuit, as a decrease in chargingcurrent occurs, the resistance of the control tube increases therebyincreasing the screen voltage to permit the charging current to increaseback to its desired value and, of course, the converse is true as thecharging current increases above a desired value. In this manner, thecharging current can be maintained at a constant value and is notinfluenced by any of the normal variables such as geometry of thescorotron, voltage variations, dirty wires, atmospheric changes, etc.,which ordinarily affects charging current.

Although scorotron devices of this type permit the obtainment of highquality reproductions having the desired contrast value on a continuousand automatic basis, recent improvements in the electrostatographicprocess enabling high speed reproductions to be obtained give rise tothe requirement of a faster charging process. The charging time requiredwith current scorotron and other charging devices presents a limitingfactor in the wide spread introduction and use of high speedreproduction techniques.

It is therefore the principle object of this invention to provideelectrical configurations for a corona generating devicewhereby auniform electrostatic charge may be rapidly deposited on anelectrostatographic plate.

It is another object of the present invention to provide a method foraccelerating the charging process of a scorotron device by applying atime varying voltage gradient to the scorotron screen.

It is still another object of the present invention to provide methodsfor obtaining the necessary voltage gradients for application to thescorotron screen.

These as well as other objects are accomplished by the present inventionwhich provides corona generating apparatus comprising a back-up plateand screen wires, at least one corona discharge wire positionedtherebetween, adapted to be mounted in closely spaced relation with anelectrostatographic plate to uniformly apply an electrostatic chargeonto said plate, a coronagenerating potential source connected to saidcorona discharge wire adapted to generate a corona discharge from saidcorona wire, and means for applying potential to said screen wiresadapted to impress a voltage gradient upon said screen wires permittinghigh initial charging currents to flow thereby rapidly charging saidplate while limiting the ultimate charge applied thereto.

In one embodiment, the corona generating apparatus contains a highvoltage transient-producing circuit adapted to be connected in serieswith the screen wires, said circuit comprising a capacitor, means forcharging said capacitor and switching means for connecting saidcapacitor in series with the screen wires whereby the capacitor ischarged, connected to said screen wires and discharged simultaneouslywith the applied potential to said screen wires from a potential sourceto provide a transient increase in the screen potential enabling rapiduniform charging of the electrostatographic plate. 1

In an alternate embodiment, a time varying voltage gradient is obtainedupon relative movement between the corona generating apparatus and theelectrostatographic plate by imposing a voltage gradient upon the screenwires decreasing in the direction of rotation of the electrostatographicplate. In this manner, the plate is initially subjected to a highvoltage which tapers down to the desired voltage as the plate rotatespast the corona charging device. The voltage gradient is convenientlyobtained by connecting each of the screen wires to taps contacting aplurality of potential dividers in series with a high voltage D. C.source, such as a battery. The leading screen wire in the direction ofrotation is connected to the potential divider at the high potential endof the series of potential dividers. Each subsequent screen wire isadapted to pick off a lower voltage with the terminal screen wire beingadapted to pick off the ultimately desired screen voltage from thepotential divider at the low potential end of the series of potentialdividers.

For a better understanding of this invention, together to the followingdetailed description of the invention to be read in connection with theaccompanying drawing wherein:

FIG. 1 is a schematic diagram of a scorotron circuit as modified by thetime varying voltage gradientproducing circuit of the present invention.

FIG. 2 is a schematic diagram of a scorotron circuit as modified by analternate position varying voltage gradient-producing circuit of thepresent invention.

The circuit illustrated in FIG. 1 is applicable for use with corotroncharging devices used in electrostatographic processes wherein there isno relative movement between the charging device and theelectrostatographic plate during charging. In the circuit illustrated inFIG. 1, the primary 10 of a stepup transformer 12 is connected to asource of alternating current, such as a commercial outlet of volts, 60cycle alternating current. One end of the high voltage secondary winding14 is connected to the anode 16 of a high voltage rectifying tube 18.The other end of secondary 14 is connected through a voltage dividingresistor or potentiometer 20 to form a rectifying circuit so thatpulsating direct current appears across the resistor 20. A smoothingcapacitor 22 is connected across the resistor 20 to smooth out thepulsations and yield a more constant direct current. The high potentialend of the potential divider 20, i.e., the end connected to the cathode24 of the diode rectifier 18 is connected by conductor 26 to theelectric connector terminal 28 of corona discharge wires 30. The lowpotential end of the potential divider is preferably grounded, as shown,and is also connected by conductor 32 to electrical connector terminal33 of the electrostatographic plate 34.

A secondary winding 36 forming a secondary of transformer 29 is providedto heat the cathode 24, although other energizing means could beprovided.

Terminal 38 of control electrode or screen wires 40 is connected byconductor 42 to a tap 44 on the potential divider 20 to charge electrode40 to a predetermined potential below the voltage of the corona wires30. It is obvious that another voltage source, such as a battery, can beused, if desired, to obtain this potential. Terminal 46 is employed toconnect shield 48 to ground or a low potential.

A potential of at least about 4,000 volts between the corona electrodeand the nearest conductors or electrodes is usually required in order togenerate a useful corona discharge and it is preferable that the voltagebe less than 10,000 volts in order to avoid sparking or excessive spacecharges in structures of practical dimensions. Either positive ornegative charge can be deposited on the electrostatographic platedepending on the polarity of the power supplies. FIG. 1 indicates thecorrect polarity for positive charging. The D. C. potential can beeither constant or it can be pulsating, such as that obtained byhalf-wave rectification of alternating current. An alternating potentialcan also be used since ions of both polarities are then made availablein alternate half-cycles.

The voltage applied between the control electrode or screen wires 40 andthe electrostatographic plate 34 is generally a constant D. C. voltageor a pulsating voltage which is synchronized with a pulsating coronavoltage.

With D. C. corona, the potential of. the control electrode or screenwires can be intermediate to the potentials of the corona electrode andthe conductive backing electrode of the electrostatographic plate or thesame as the potential of the conductive backing. In any event, thevoltage between the control electrode and the conductive backing of theelectrostatographic plate is kept below the breakdown voltage of theinsulating layer to be charged. With electrostatographic plates, thisvoltage usually will not exceed 1,000 volts.

' With D. C. corona, the polarity of charge applied to the plate will bethe same as the polarity of the corona electrode regardless of thepolarity of the control electrode. With A. C. corona the polarity of thecharge applied to the insulating layer will be the same as that of thecontrol electrode.

In accordance with the present invention, it has been found that theelectrostatographic plate current is generally proportional to thepotential of the screen wires or control electrode. Thus, the chargingcurrent can be considered as being proportional to the differencebetween the scorotron screen potential and the potential on the surfaceof the electrostatographic plate. Through the present invention, a timevarying potential gradient is imparted to the electrostatographic plateby means of the scorotron screen, enabling more rapid charging of theelectrostatographic plate. In the embodiment shown in FIG. 1, the timevarying potential is obtained by connecting a charged capacitor 50 inseries with the supply voltage to the scorotron screen. A high voltagetransientproducing circuit comprises capacitor 50, switch 52 and a highvoltage D. C. power source, such as battery 54. The capacitor is chargedwhen the switch is in position a. In this position, the scorotron actsin the conventional manner. However, when extra charging speed isrequired, switch 52 is switched into position b in an appropriate timesequence with the application of the supply voltage to the screen. Inthis manner, the magnitude of the increased screen voltage is determinedby the potential of battery 54 and the duration of the increased screenpotential is determined by the time constant which is equal to RC.Preferably, the product of the screen current and the resistance R issmall as compared to the high voltage to afford accuracy in setting thescreen voltage.

In operation, for example, if it is desired to charge a sample to -300volts, the conventional screen setting would be 300 volts. According tothe present invention, however, a high voltage transient of, forexample, 800 volts could be impressed upon the screen by allowing thecapacitor 50 to discharge in series with the supply voltage therebyimparting a high voltage transient for the duration of the timeconstant. Thereafter the screen would return to its conventional 300volts. In this manner, larger initial charging currents are permitted toflow and therefore charge the sample more rapidly.

The embodiment shown in FIG. 2 is suitable for use with automaticrecyclable electrostatographic copying machines.

The automatic electrostatographic reproducing apparatus generallycomprises an electrostatographic plate 100 including a photoconductivelayer or light receiving surface on a conductive backing, formed in theshape of a drum, which is mounted on a shaft journaled in a frame torotate in the direction indicated by the arrow to cause the drum surfacesequentially to pass a plurality of electrostatographic processingstationsduring the reproduction cycle.

The scorotron charging device generally designated is situated above theelectrostatographic plate and is adapted to deposit a uniformelectrostatic charge thereon upon rotation of the plate past thecharging device.

In the circuit illustrated in FIG. 2, a high voltage is applied to thecorona discharge wires 112 in the same manner as described in referenceto FIG. 1. The backing shield 114 and the electrostatographic plate 100are connected to ground or a low potential. The screen wires shownrepresentatively as 116a, l16b and 1160 are respectively connected viaconductors 118, 120 and 122 to taps 124, 126 and 128 on a plurality ofpotential dividers 130, 132 and 134 connected in series with a highvoltage D. C. source such as battery 136. The low potential end of theseries of potential dividers is preferably grounded as shown. In thismanner, a multiplicity of voltages is conveniently impressed upon thescreen wires. Upon relative movement between the screen wires and theelectrostatographic plate, a time varying voltage gradient is generatedat each point on the electrostatographic plate in the direction ofrotation enabling rapid uniform charging of the plate.

In operation, if it is desired, for example, to charge a sample to 300volts, the conventional screen setting would be 300 volts. According tothis embodiment of the present invention, however, a multiplicity ofvoltages is imposed upon the screen wires, for example, by connectingthe lead screen wire 116a in the direction of rotation of the plate 100to the potential divider 130 at the high potential end of the series ofpotential dividers. The tap 124 can be set to draw 800 volts, forexample, from potential divider 130. The next screen wire l16b in thedirection of rotation of the plate can be connected to potential divider132 through tap 126 set, for example, at an intermediate value of 550volts. The last screen wire ll6c in the direction of rotation can beconnected to the potential divider 134 at the low potential end of theseries via tap 128 set at the desired sample voltage of -300 volts. Inthis manner, upon relative movement between the charging device and therotating plate, every point on the plate surface is exposed to a timevarying voltage gradient permitting larger initial charging current toflow thereby charging the sample more rapidly.

While the invention has been described with reference to the circuitsdisclosed herein, it is not confined to the detail set forth since it isapparent that certain electrical equivalent components may besubstituted for components of the preferred circuits without departingfrom the scope of the invention. It is readily apparent, for example,that although power sources 54 in FIG. 1 and 136 in FIG. 2 are shown asbeing independent power sources, they need not be. The requiredpotential can easily be obtained by appropriate tapping of the mainpower supply such as by tapping the potential divider 20 in FIG. 1. Thisinvention is therefore intended to cover such modifications or changesas may come within the purposes of the invention as defined by thefollowing claims.

What is claimed is:

1. Corona generating apparatus comprising a backup plate and screenwires, at least one corona discharge wire positioned therebetween,adapted to be mounted in closely spaced relation with anelectrostatographic plate to uniformly apply an electrostatic chargeonto said plate, a corona-generating potential source means connected tosaid corona discharge wire adapted to generate a corona discharge fromsaid corona wire, and a second potentialsource means coupled to saidscreen wires for applying to said screen wires a potential having awaveform which continuously varies in amplitude for causing an initiallyhigh charging current to flow to said plate and to vary in amplitudefrom a high initial amplitude to a lower amplitude thereby rapidlycharging said plate while limiting the ultimate charge applied thereto.

2. Corona generating apparatus comprising a backup plate and screenwires, with at least one corona discharge wire positioned therebetween,adapted to be mounted in closely spaced relation to anelectrostatographic plate to uniformly apply an electrostatic chargeonto said plate; a corona-generating potential source connected to saidcorona discharge wire adapted to generate a corona discharge from saidcorona wire, a second potential source connected to said screen wiresadapted to limit the potential applied to said electrographic plate bysaid corona discharge, a high potential transient-producing circuitadapted to be connected in series with the screen wires, saidtransient-producing circuit comprising a capacitor, means for chargingsaid capacitor and switching means for connecting said capacitor inseries with the screen wires whereby the capacitor is charged, connectedto said screen wires and discharged in sequence with the application ofpotential to said screen wires to provide a transient increase in thescreen potential enabling rapid charging of the electrostatographicplate.

3. Circuitry for generating a corona discharge adjacent to anelectrostatographic plate for rapidly and uniformly applying anelectrostatic charge onto said plate including a source of directcurrent having a potential divider in said source and a common return,

at least one corona discharge wire positioned in close proximity to saidelectrostatographic plate adapted to apply an electrostatic charge ontosaid plate,

means coupling said corona discharge wire to said direct current at arelatively high potential,

at least one screen wire positioned between said corona discharge wireand said electrostatographic plate and said screen wire coupled to saiddirect current, at a potential reduced in amplitude with respect to saidrelatively high potential,

a back-up plate positioned adjacent said corona discharge wire on theopposite side thereof from said screen wire, said back-up plate coupledto said common return,

means coupling said electrostatographic plate to said common return,

means for applying a potential which varies continuously in amplitude,with respect to said common return during a charging interval, to saidscreen wire in series with said reduced potential for regulating thecharge being applied to said elec- 5ili $1fi ?$%m% nai?si3l32? ifl%%i%tial, such that a corona discharge having a high initial value, and alower terminating value is applied to the electrostatographic plate,thereby enabling the rapid and uniform charging of said plate to saidlower terminating value.

4. Circuitry for generating a corona discharge asin claim 3 and in whichsaid means for applying a varying potential includes an R-C chargingcircuit and a multi-position switching means arranged for coupling theR-C charging circuit to a charging potential, in one switch position andin another switch position for coupling a capacitor of the R-C chargingcircuit to said screen wire for discharging said capacitor through saidscreen wire circuit thereby varying the potential applied to said screenwire during discharge of said capacitor.

5. A fast charging corona generating apparatus for charging anelectrostatographic plate comprising:

a discharge assembly having a back-up plate, a screen grid, and at leastone corona discharge wire positioned between said plate and grid;

said assembly adapted to be mounted in closely spaced relation withrespect to an electrostatographic plate for uniformly establishing anelectro static charge of potential V on said plate;

a source of corona generating potential coupled to said corona dischargewire for establishing a corona discharge from said wire; and,

circuit means coupled to said screen grid for applying to said screengrid a potential which varies continuously in amplitude during at leasta portion of a charging interval from an amplitude V which causes theflow of an initially high charging current to said plate, to anamplitude V 6. The apparatus of claim 5 wherein said circuit means isadapted for applying to said screen grid a D. C. potential of amplitudeV and for applying in series therewith a potential which variescontinuously between the amplitudes V and V 7. The apparatus of claim 5wherein said circuit means provides a screen grid potential which variesexponentially between the amplitudes V and V,.

8. The apparatus of claim 7 wherein said circuit means for providing anexponentially varying amplitude includes an RC. discharge circuit.

9. The apparatus of claim 8 wherein switching means are provided foralternatively coupling and decoupling said R.C. circuit in series withthe potential V applied to said screen grid.

1. Corona generating apparatus comprising a back-up plate and screenwires, at least one corona discharge wire positioned therebetween,adapted to be mounted in closely spaced relation with anelectrostatographic plate to uniformly apply an electrostatic chargeonto said plate, a corona-generating potential source means connected tosaid corona discharge wire adapted to generate a corona discharge fromsaid corona wire, and a second potential source means coupled to saidscreen wires for applying to said screen wires a potential having awaveform which continuously varies in amplitude for causing an initiallyhigh charging current to flow to said plate and to vary in amplitudefrom a high initial amplitude to a lower amplitude thereby rapidlycharging said plate while limiting the ultimate charge applied thereto.2. Corona generating apparatus comprising a back-up plate and screenwires, with at least one corona discharge wire positioned therebetween,adapted to be mounted in closely spaced relation to anelectrostatographic plate to uniformly apply an electrostatic chargeonto said plate; a corona-generating potential source connected to saidcorona discharge wire adapted to generate a corona discharge from saidcorona wire, a second potential source connected to said screen wiresadapted to limit the potential applied to said electrographic plate bysaid corona discharge, a high potential transient-producing circuitadapted to be connected in series with the screen wires, saidtransient-producing circuit comprising a capacitor, means for chargingsaid capacitor and switching means for connecting said capacitor inseries with the screen wires whereby the capacitor is charged, connectedto said screen wires and discharged in sequence with the application ofpotential to said screen wires to provide a transient increase in thescreen potential enabling rapid charging of the electrostatographicplate.
 3. Circuitry for generating a corona discharge adjacent to anelectrostatographic plate for rapidly and uniformly applying anelectrostatic charge onto said plate including a source of directcurrent having a potential divider in said source and a common return,at least one corona discharge wire positioned in close proximity to saidelectrostatographic plate adapted to apply an eLectrostatic charge ontosaid plate, means coupling said corona discharge wire to said directcurrent at a relatively high potential, at least one screen wirepositioned between said corona discharge wire and saidelectrostatographic plate and said screen wire coupled to said directcurrent, at a potential reduced in amplitude with respect to saidrelatively high potential, a back-up plate positioned adjacent saidcorona discharge wire on the opposite side thereof from said screenwire, said back-up plate coupled to said common return, means couplingsaid electrostatographic plate to said common return, means for applyinga potential which varies continuously in amplitude, with respect to saidcommon return during a charging interval, to said screen wire in serieswith said reduced potential for regulating the charge being applied tosaid electrostatographic plate by said corona discharge wire to a valueproportional to said varying potential, such that a corona dischargehaving a high initial value, and a lower terminating value is applied tothe electrostatographic plate, thereby enabling the rapid and uniformcharging of said plate to said lower terminating value.
 4. Circuitry forgenerating a corona discharge as in claim 3 and in which said means forapplying a varying potential includes an R-C charging circuit and amulti-position switching means arranged for coupling the R-C chargingcircuit to a charging potential, in one switch position and in anotherswitch position for coupling a capacitor of the R-C charging circuit tosaid screen wire for discharging said capacitor through said screen wirecircuit thereby varying the potential applied to said screen wire duringdischarge of said capacitor.
 5. A fast charging corona generatingapparatus for charging an electrostatographic plate comprising: adischarge assembly having a back-up plate, a screen grid, and at leastone corona discharge wire positioned between said plate and grid; saidassembly adapted to be mounted in closely spaced relation with respectto an electrostatographic plate for uniformly establishing anelectrostatic charge of potential V1 on said plate; a source of coronagenerating potential coupled to said corona discharge wire forestablishing a corona discharge from said wire; and, circuit meanscoupled to said screen grid for applying to said screen grid a potentialwhich varies continuously in amplitude during at least a portion of acharging interval from an amplitude V2, which causes the flow of aninitially high charging current to said plate, to an amplitude V1. 6.The apparatus of claim 5 wherein said circuit means is adapted forapplying to said screen grid a D. C. potential of amplitude V1 and forapplying in series therewith a potential which varies continuouslybetween the amplitudes V2 and V1.
 7. The apparatus of claim 5 whereinsaid circuit means provides a screen grid potential which variesexponentially between the amplitudes V2 and V1.
 8. The apparatus ofclaim 7 wherein said circuit means for providing an exponentiallyvarying amplitude includes an R.C. discharge circuit.
 9. The apparatusof claim 8 wherein switching means are provided for alternativelycoupling and decoupling said R.C. circuit in series with the potentialV1 applied to said screen grid.